Ca plays a pivotal role in both excitation-contraction (EC)-coupling and activation signaling pathways in the myocardium. One of the most significant remaining unanswered questions in cardiac biology is how Ca- dependent signaling pathways are regulated against the backdrop of the Ca transients that mediate contraction. The working hypothesis of this proposal is that activation of Ca signaling by Ca influx through the orphan functional T-type Ca channel (TTCC) into a spatially constrained microdomain is centrally involved in the generation of new cardiac myocytes in the normal and diseased heart. We will specifically test the novel hypothesis that ventricular myocyte proliferation and the regenerative capacity of the heart is regulated, in part, by the action of TTCCs. Ca influx through TTCC is proposed to permit and/or directly enhance total myocyte numbers in the heart, reflecting the known centrality of Ca as a regulator of proliferation in nearly all cells evaluated to date. To examine this hypothesis in a rigorous manner the expertise of a genetics/signaling-based laboratory will be combined with that of an electrophysiologic/Ca regulation-based laboratory as an integrated approach. The Specific Aims of the proposal are: 1). To determine if enhanced TTCC activity in the heart can increase ventricular myocyte numbers, cell cycle activity, and myocardial recovery after injury. These studies will be performed in cardiac-specific transgenic mice inducibly expressing the alphal G or alphal H pore forming subunits of the TTCC. The extent of new myocyte formation will be inferred using a unique, myocyte-specific genetic recombination system in vivo, as well as with classic cell-based and histological techniques. Direct measurements of total ventricular myocyte numbers will be measured at baseline and after injury to assess the extent to which TTCC activity affects the regenerative capacity of the heart. 2). To determine if reduced TTCC activity reduces ventricular myocyte numbers in the heart, cell cycle activity, and impairs myocardial regeneration after injury. Mice lacking the two major cardiac T-type current generating gene isoforms, alphal G and alphal H, will be analyzed as neonates, adolescents, and adults as described above. These studies will define fundamental roles of TTCC in the generation of new ventricular myocytes and should identify novel approaches to repair damaged hearts by increasing new cardiac muscle formation.